Method for a formation properties determination

ABSTRACT

The method for a productive formation properties determination comprises positioning a complex well-logging tool in a borehole, the well-logging tool consists of the device for the formation temperature impact and two similar logging probes located symmetrically along the well-logging tool relative to the device for the formation temperature impact. During the logging tool movement in the borehole continuous formation temperature impact and formation temperature measurement are performed. Based on the obtained dependencies of the formation parameters in question as a function of temperature the productive formation properties are determined.

FIELD OF THE INVENTION

The invention is related to the geophysical well logging (GWL),particularly to the methods of productive formation study using themethod of well logging with the formation temperature exposure.

The essence of the GWL methods consists in the borehole longitudinalmeasurement of a certain value characterizing one physical property or aset of physical properties of the rocks crossed with boreholes inpressure-temperature conditions determined by the rock occurrence depthand different geological and technological conditions.

BACKGROUND OF THE INVENTION

Currently more than 50 GWL methods and their modifications are known. Adisadvantage of the known methods consists in the fact that themeasurement of the physical values at certain rock occurrence depth isperformed at certain pressure-temperature conditions not subject tochanges during the logging.

From the prior art, methods of the rock properties determination duringthe formation thermal exposure are known. Thus, USSR Certificate ofAuthorship No. 1125519 describes a method of the productive formationsdetermination where the deposit is exposed to the thermal effect andnuclear-magnetism logging or acoustic logging is performed before thethermal exposure and after it. Free fluid index, spin-lattice relaxationand porosity is measured and based on these values oil recovery factoris evaluated. Hereby the formation thermal regime is set by means ofinjecting a heating agent or by means of fireflooding.

The closest prior art method is the method of the formation parametersdetermination described in U.S. Pat. No. 6,755,246, in which theformation active or passive heating is performed in order to increasethe formation fluid temperature by means of which relaxation time T₂ ismeasured with the spin echo measurement which is used to identify andquantify the heavy oil saturation. This method's disadvantages includethe fact that it is performed using “logging-exposure-logging” patternwhich significantly increases the GWL time.

One of the nuclear magnetism logging method disadvantages consists inthe fact that the decay time constant in some formations, for example,in low-permeability sands is very low which does not permit the signalsmeasurement. The main problem linking the relaxation time and theformation permeability is the fact that the pores studied using nuclearmagnetism resonance method must not necessarily be hydraulicallyinter-connected. Consequently, an impermeable medium includingstand-alone voids may yield the same T₁ decay curve as a permeable rockincluding inter-connected pores.

SUMMARY OF THE INVENTION

The claimed invention provides for the improved data collection usingGWL methods, enhanced technological and functional features of the GWLequipment.

The method comprises positioning a complex well-logging tool in aborehole, the well-logging tool consists of a device for the formationtemperature impact and two identical logging probes locatedsymmetrically along the well-logging tool length relative to the devicefor the formation temperature impact, well logging with the simultaneouscontinuous formation temperature impact and formation temperaturemeasurement during the probe displacement in the borehole. If necessaryat least one repeated logging of the well may be performed. Theproductive formation properties are determined based on the obtainedvalues of the parameters in question as a function of the formationtemperature.

The formation temperature impact is performed by means of the heating orcooling thereof. Heating may be performed by means of the downholeheater. Any logging type, for example, acoustic logging, electricallogging etc. may be used. The oil formation properties to be determinedare formation relative permeability and/or saturating fluid viscosityand/or viscous flow activation energy. The complex well-logging tool mayinclude at least one additional formation temperature impact device andat least one additional logging probe mounted alternately and located insuch a way that each additional formation temperature impact device islocated between two identical logging probes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is explained by the drawing where FIG. 1 shows an exampleof the claimed invention implementation.

The method for the formation properties determination as per thisinvention is implemented as follows.

Well-logging tool 2 including one or more devices 3 for the temperatureimpact as well as two or more identical logging probes 4 locatedsymmetrically along the tool 2 length relative to the device 3 for thetemperature impact on the rock mass area in question are delivered intothe borehole 1 using any method possible and, using tool 2, the welllogging is performed. The logging is performed continuously during thetool upward displacement in the borehole. Simultaneously, during thedisplacement temperature impact on the area in question is performed andthe area temperature is measured.

After that at least one repeated logging is performed, during eachsinking/hoisting operation the formation temperature is measured. Curvesof the parameter in question vs., for example, in case of acousticlogging, velocity and decay of the acoustic waves generated by thelogging probe are built, and using the curves obtained the judgment ofthe formation parameters is made.

The productive formation properties to be determined may includeformation relative permeability, saturating fluid viscosity, viscousflow activation energy.

As a specific example of the method implementation the application ofthe acoustic logging based on the measurement of the temperaturedependencies of the dispersion curves for Stoneley wave is shown. Theinvention example is based on the fact that in permeable formationsStoneley wave amplitudes undergo frequency-dependent decay resultingfrom the fluid flow. Non-elastic decay degree is proportional to theformation fluid mobility. Analysis of Stoneley waves generated indifferent temperature conditions is a permeable formation permeabilityindicator as well as saturating fluid viscosity indicator.

A well-logging tool including a device for the temperature impact on therock mass area in question (the device provides the rock mass areatemperature measurement) as well as two or more identical acousticlogging probes located symmetrically relative to the device for thetemperature impact are delivered into the borehole using any methodpossible. The logging is performed continuously during the tool upwarddisplacement in the borehole, simultaneously the temperature exposureresulting in the temperature change is performed. The temperatureexposure of the formation may be performed by the heating or coolingthereof. The formation may be heated using a local downhole heater.Stoneley waves' velocity and decay values are measured with theoperating local heater to change the temperature of the rock mass inquestion and measure the formation temperature. Symmetrical positioningof the acoustic logging probes relative to the downhole heater enablesStoneley waves' velocity and decay measurements before, during and afterthe heater exposure of the area in question. Let us consider a solidporous formation in which a borehole is drilled. The porous space andborehole are filled with a viscous two-phase fluid. Stoneley wavevelocity and decay are some of the information parameters determinedusing acoustic logging methods.

Stoneley wave velocity V_(t) in this borehole is determined by theequation:

$\begin{matrix}{{V_{t}^{- 2} = {\rho_{f}\left\lbrack {\frac{1}{K_{f}} + \frac{1}{G} - {\frac{2}{\; a\; \omega}\frac{k_{0}}{\eta_{f}}{E\left( {a\sqrt{{- {\omega}}/D}} \right)}}} \right\rbrack}},} & (1)\end{matrix}$

where ω is cyclic frequency, ρ_(f), η_(f), K_(f)—density, viscosity andbulk modulus of elasticity of the porous fluid; K, G, k₀—bulk modulus ofelasticity, shear modulus and absolute permeability of the formation,φ—formation porosity, a—borehole radius; E(x)=xK₁(x)/K₂(x), hereK_(0,1)(x) are Kelvin functions; D—diffusion coefficient for the Biotwave of the second kind:

$\begin{matrix}\begin{matrix}{D = {\frac{k_{0}}{\eta_{f}}\frac{K_{f}}{\varphi}\left( {1 + \xi} \right)^{- 1}}} \\{\approx {\frac{k_{0}}{\eta_{f}}\frac{K_{f}}{\varphi}}} \\{{= {\frac{K_{f}}{\varphi}{k_{0}\left( {\frac{k_{1}}{\eta_{1}} + \frac{k_{2}}{\eta_{2}}} \right)}}},}\end{matrix} & (2)\end{matrix}$

correction ξ for rigid formations may be neglected (K+(4/3)G>>K_(f)).Here viscosities η_(1,2) and relative phase permeabilities k_(1,2) forliquid phases filling the porous space were introduced.

Stoneley wave velocity determined by the equation (1), is a complexvalue, to obtain phase velocity c_(t) and decay rate α, it is necessaryto segregate real and imaginary part of this equation:

c _(t) ⁻¹ =Re(V _(t) ⁻¹), α=ωIm(V _(t) ⁻¹).

Measuring frequency dependency of the Stoneley wave phase velocityc_(t)(ω) or decay α(ω) for different temperatures and using non-linearsimplex approximation method for the model (1) it is possible todetermine the temperature dependency for diffusion coefficient D(T). Theonly temperature-dependent values included in the equation (2)determining diffusion coefficient D are viscosities of the liquid phasesfilling the porous medium. Liquid phase vs. Temperature dependency canbe approximated using Arrhenius law with a good accuracy:

η_(1,2)=η_(1,2) ⁰ exp(W _(1,2) /RT),  (3)

where W_(1,2) are viscous flow activation energy values, T—absolutetemperature, R—universal gas constant. If the activation energy valuesW_(1,2) are different, the contributions of the fluids into thediffusion coefficient will change as the temperature changes. With theknown porosity φ and absolute permeability k₀ values it allows using thesystem of two linear equation in the form of (2), written for twodifferent formation temperatures to determine unknown values of therelative phase permeability k_(1,2).

The solution to this system looks as follows:

${\begin{bmatrix}k_{1} \\k_{2}\end{bmatrix} = {\frac{\varphi}{k_{0}K_{f}\Delta}\begin{bmatrix}{\left( {\eta_{1}^{e}/\eta_{2}^{e}} \right)\left( {{\eta_{2}^{b}D^{b}} - {\eta_{2}^{e}D^{e}}} \right)} \\{\left( {\eta_{2}^{b}/\eta_{1}^{b}} \right)\left( {{\eta_{1}^{e}D^{e}} - {\eta_{1}^{b}D^{b}}} \right)}\end{bmatrix}}},$

Where superscripts ‘b’ and ‘e’ denote initial and final temperaturerespectively, the system determinant is equal to:

$\Delta = {{{\frac{\eta_{2}^{b}}{\eta_{1}^{b}}\frac{\eta_{1}^{e}}{\eta_{2}^{e}}} - 1} = {{{\exp \left( {\frac{\Delta \; W}{R}\left( {\frac{1}{T^{b}} - \frac{1}{T^{e}}} \right)} \right)} - 1} \approx {{\exp \left( {\frac{\Delta \; W}{{RT}^{2}}\Delta \; T} \right)} - 1.}}}$

This equation system will be conditioned the better (and, consequently,the experimental data processing error will be the smaller), the higherthe value of the liquid phases activation energy values' differenceΔW=W₂−W₁ is, namely when the following condition is met:

$\begin{matrix}{{\ln \left( {\Delta - 1} \right)} \approx {\frac{\Delta \; W}{{RT}^{2}}\Delta \; T} \geq 1.} & (4)\end{matrix}$

Admitting that the pore fluid is a mixture of water and oil we willdetermine the value by which the activation energy for water must behigher in case of the pore fluid heating by 30° C. Supposing that theformation temperature is 330 K, we obtain the value of ΔW=30 kJ/mol. Forthe water activation energy is equal to W₁=19.3 kJ/mol, consequently,the methodology offered will work well for oil fields where theactivation energy value is comparable with 50 kJ/mol—as a rule, this ischaracteristic for viscous or heavy oil fields.

As per the method proposed logging probe acoustic waves' velocity anddecay oscillations are measured relative to the relevant temperaturechange generated due to the local downhole heater operation. In thepreferable embodiment of the invention one and/or more heaters arelocated between two and/or more identical acoustic probes due to whichfact the measurements are performed at least twice—using one probebefore the heating and the other one during the heating. Then themeasured parameters corresponding to different temperatures of the rockmass in question between different pairs of acoustic probes areanalyzed. As a result, by the set dependencies of the logging probeacoustic waves' velocity and decay as function of temperature as well asby the dependencies of saturating fluid viscosity as function oftemperature relative phase permeability values, saturating fluidviscosity and viscous flow activation energy may be determined.

1. A method for the determination of a formation relative phasepermeability values comprising the steps of: disposing a complexborehole logging tool in a borehole, the tool comprising a device forthe formation temperature impact and at least two identical acousticlogging probes located symmetrically along the tool length relative tothe device for the formation temperature impact, performing during thetool movement in the borehole the continuous acoustic logging with thesimultaneous continuous formation temperature impact and measurement ofthe formation temperature as well as velocity and decay the Stoneleywaves generated by the logging probe, and determining the formationrelative phase permeability values based on the obtained dependencies ofthe Stoneley waves' velocity and decay as a function of temperature. 2.The method of claim 1 wherein the formation temperature impact isperformed by heating the formation.
 3. The method of claim 1 wherein theformation temperature impact is performed by cooling the formation. 4.The method of claim 2 wherein the heating is performed using a downholeheater.
 5. The method of claim 1 wherein at least one repeated loggingis performed.
 6. The method of claim 1 wherein the logging toolcomprises at least one additional device for the formation temperatureimpact mounted alternately and positioned in such a way that eachadditional device for the formation temperature impact is positionedbetween two similar logging probes.